Interior concrete slab guide: vapor, moisture, and flatness

Last updated: August 20, 2026

An exterior slab is the finish. An interior concrete slab is a substrate for one, and that changes how it gets built. Three numbers govern the outcome: a 10 to 15 mil vapor barrier under the concrete, a moisture reading under 3 pounds per 1,000 square feet, and FF 50 flatness. Miss one and the correction costs more than the discipline would have. Thickness, base, and mix fundamentals live in the concrete flatwork guide.

Why an interior slab takes more discipline than a driveway

Nobody polishes a driveway. Exterior flatwork gets a broom finish, no vapor barrier, and flatness tolerances loose enough that nobody measures them, because the surface is the product and the weather is the only critic. A polished interior floor gets held to FF 50 and a moisture reading under 3 pounds. Same truck, different work.

Indoors, the slab has to hand off to something: a coating, a polish, tile, vinyl, adhesive. Moisture that would evaporate harmlessly off a patio has nowhere to go once a floor covering seals the top. Ripples that nobody notices on a garage apron read like waves under a showroom’s sidelight. The surface, the moisture control, and the flatness all matter in ways they simply do not outside.

So the finishing changes too. The exterior slab gets a textured broom finish because texture is traction and nobody is going to grind it. The interior slab gets a steel trowel, a sealed barrier under it, a tight flatness tolerance, and a water cure that leaves no bond-breaking film behind. Same concrete, same crew, completely different set of decisions from the dirt up.

Does an interior concrete slab need a vapor barrier?

Yes, and this one is not negotiable. Any slab inside a conditioned space gets a 10 to 15 mil polyethylene sheet laid directly under the concrete, on top of the granular base, before the pour. Seams lap six inches and get taped. Every plumbing and conduit penetration gets sealed.

The sheeting costs a few hundred dollars on a typical home slab. Here is what skipping it buys instead: epoxy and polyaspartic coatings that delaminate, tile and vinyl adhesives that release, efflorescence that stains the surface white, and a musty smell nobody can find the source of. Ground moisture does not stop moving because a slab got poured over it. It migrates up, and whatever you put on top becomes the ceiling it pushes against.

The sand blotter layer is a myth worth killing

There is an old detail where a few inches of sand goes between the vapor barrier and the concrete, meant to give bleed water somewhere to go. It does the opposite. That sand layer fills with water from the mix, from rain during construction, from anything, and the barrier underneath means it has no escape route downward. So it sits there and pumps moisture into the slab from below for years.

The barrier goes directly under the slab. Not under sand. If a plan sheet still calls for a blotter layer, that is the conversation to have before the pour, not after the floor gets coated.

How do you test an interior slab for moisture before finishing?

Two standardized tests settle it, and they measure different things. ASTM F1869, the calcium chloride test, sets a measured dish of anhydrous calcium chloride sealed on the slab for 60 to 72 hours; the weight it gains converts to a moisture vapor emission rate in pounds per 1,000 square feet per 24 hours. ASTM F2170 drills relative humidity probes to 40 percent of the slab depth and reads what the concrete is doing inside, which matters because a thick slab’s surface can read dry while the core is still wet.

The widely cited acceptance number is 3 pounds per 1,000 square feet per 24 hours, and some coating systems tighten it further. New concrete typically needs 28 days or more before it reads that low, which is worth knowing when a general contractor asks whether the floor can be coated next week.

A slab reads dry to the hand long before it reads dry to a probe. That gap is where most coating failures are born.

The order also matters. The test happens before the coating goes down, not after the floor starts letting go, because at that point the number only tells you why you are paying twice. On a schedule where the finish date is fixed and the pour date keeps sliding, the moisture reading is the one line item that cannot be compressed to make up the difference.

What FF and FL actually measure

FF is floor flatness and FL is floor levelness, both defined by ACI 117 and both read with a profiler such as a Dipstick or an inclinometer cart. FF describes bumpiness over short distances. FL describes overall tilt across the room. They are independent: a slab can be dead level and still ripple, or dead flat and running downhill.

A standard troweled pour lands around FF 25 to 30. A slab destined for polished concrete wants FF 50 or higher. That gap is the difference between grinding to refine a surface and grinding to reshape one.

Flatness is cheap at pour time and expensive afterward. Every point of FF you buy with a laser screed, a good finish crew, and honest time on the trowel is a point you do not buy back with a diamond and a week of labor. And you never fully buy it back, because grinding a wavy slab flat means cutting deep into the cream and sometimes into the aggregate, which changes the look of the floor whether you wanted that or not.

What sits under the slab

The standard stack, bottom to top: compacted subgrade, 4 to 6 inches of crushed granular base, vapor barrier, then concrete. Every layer has one job, and compaction is the one people rush because it does not show up in a photo.

Two situations add a layer. A heated slab with radiant PEX tubing cast into the concrete needs rigid foam insulation under it, or you are paying to warm the dirt. Cold-climate slabs need the same foam for the same reason. Use extruded or expanded polystyrene rated to at least 25 psi compressive strength, because foam that crushes under the pour is worse than no foam at all: now you have a soft spot and a low spot in the same place.

Prep is different for polish than it is for coating

Both finishes start from the same slab, but they want it handed over in different conditions.

Polished concrete wants a hard-troweled, FF 50 or better slab with a low water-to-cement ratio. Dense surface, tight cream, no film on top. That is why a water cure matters here: it leaves a clean surface the diamonds can bite, while an acrylic curing compound has to be ground off entirely before the polishing sequence can even start.

Epoxy flooring and other coatings want mechanical tooth instead. The crew diamond-grinds or shot-blasts the slab to a concrete surface profile inside the range the coating maker specifies, and the moisture reading is the gate: under the maximum MVER or RH the system allows, or the coating does not go down that day.

The four mistakes that cost the most

A sand blotter layer over the vapor barrier. It traps water with no way out and feeds the slab from below for the life of the building.

Skipping the moisture check because the slab looks dry. Looking dry is a surface condition. Coating over a wet core is how a floor delaminates six months after everyone signed off.

Acrylic curing compound used for convenience. It cures the slab and breaks the bond, and every square foot has to come back off before a finish will stick.

Accepting a standard FF 25 pour for a floor meant to be polished. That decision guarantees grinding overruns and a wavy reflection, and it gets made months before anybody notices, usually by somebody who was never told the floor would be polished. The concrete flatwork guide covers the pour-side fundamentals that keep this from happening.

Full scope, pricing, and the specification tiers for the pour itself are on the interior slab service page. Every one of these decisions is cheap on the day it gets made and expensive on every day after that.